Back to the Future is a fantastic sci-fi comedy, in which a teenager, Marty McFly (Michael J. Fox), travels back in time in a car, which has been made into a time-travel machine. Marty lands in 1955, just before his parents, then teenagers, are supposed to meet. Marty does all he can to make that happen - otherwise he won't get born! Another problem is how to get back to 1985.
The film especially succeeds in its ironic play on the contrasts between the two ages.